FIG. 1 shows a prior art wireless packet 100 which includes a preamble 102 part and a frame 104 part, which frame 104 may further comprise a header and payload. The preamble part 102 is present for an interval of time T1 106, and the frame part is present for an interval of time T2 108. A wireless receiver which receives the wireless packet performs a packet detection step based on detection of the symbols of the preamble 102, and also performs a first AGC operation such that the gain of the receiver is modified to place the signal energy of the packet in a best operating range for the analog to digital converters (ADC) which are sampling the signal after conversion to baseband by an RF front end.
FIG. 2 shows an example prior art single conversion wireless receiver for a single receive channel, where incoming signals are received by antenna 202 and applied to low noise amplifier 206 having a low noise figure, with the preamplifier 206 coupled to quadrature mixers 220 and 228, which heterodyne the amplifier 206 output with a first local in-phase (I) oscillator 224 and a second local quadrature (Q) oscillator 230 with the mixer outputs coupled to low pass filters 208 and 210, respectively, to remove any image frequencies, after which the filter 208 and 210 outputs are provided to variable gain amplifiers 212 and 214, which are provided to anti-aliasing filters 216 and 218 to produce baseband outputs 252 and 254. An AGC controller 264 examines the I 248 and Q 250 receiver outputs after analog to digital (ADC) conversion 242 and 246, and generates one or more gain control outputs shown as 238. The AGC controller 264 is active during the preamble 102 part of a packet, with the general characteristic of applying a sufficiently high gain level to the variable gain amplifiers of RF receiver 204 to enable the detection of low level packets, and upon detection of a large level signal at the A/D outputs 248 and 250, reducing the RF front end 204 gain until the preamble signal is in the usable range of the A/D converters 242 and 246. The baseband signal processor 240 also includes a function for detecting start of packet, typically based on correlating the preamble with a delayed conjugated copy of the preamble which derives a preamble signal part and comparing the preamble signal part to the auto-correlation of the preamble with its conjugate, thereby deriving a signal plus noise part. Packet detect may be performed by comparing a ratio or difference of the signal part to the signal plus noise part with respect to a threshold. For wireless protocols such as 802.11b and 802.11g, the preamble is a time-domain BPSK modulated barker code which repeats over several cycles, and for 802.11, 802.11a, and 802.11e, the preamble is an OFDM subcarrier-based symbol which repeats over several cycles, such that delayed autocorrelation or cross correlation of the received preamble signal with a preamble symbol conjugate or a delayed conjugated copy of the received preamble will generate an increased correlation value at the point of repeated preamble, such that the start of packet and packet synchronization functions may be accomplished. The process of detecting the particular point in time of increased correlation for a repeated part of the preamble is known as “packet detection”.
Another variation of prior art FIG. 2 is a configuration for receiving RF signals with multiple antennas, particularly in the field of multiple input multiple output (MIMO) receivers. For MIMO receivers, multiple instances of RF Front End 204 are employed, each connected to its own antenna 202 and generating a unique stream of quadrature outputs 248, 250, and each with its own AGC controller 264. The process of AGC gain control in combination with packet detection is known as packet acquisition, as it provides the requisition conditions of gain control and identification of the preamble of the packet, both of which are required for the packet to be received and demodulated.
One of the challenges of a baseband processor is the detection of low power signals, particularly those near the noise threshold of the receiver. Accordingly, one of the performance metrics of the wireless receiver is sensitivity, which is defined as the signal strength at which a packet can be received. The capability of various receivers are differentiated by the associated receiver sensitivity metric.
In the prior art method of packet acquisition, AGC gain control is followed with packet detection. In this method, the minimum receiver sensitivity (the minimum incoming signal strength for which the packet is successfully received) is limited by the receiver gain, as determined by the AGC processor 264. Only when the AGC controller 264 generates a suitable gain level can the subsequent baseband functions of converting the received packet into data to place in a packet buffer be successful, largely because the receiver might not be able to receive packets at very low SNRs, such as those less than 3 dB SNR. An additional complication is false packet detection in the MIMO configuration, where an interfering signal or a signal from a weak baseband stream causes a false packet detection. False packet detection is any event which results in the incorrect identification of a possible preamble by the packet detection circuit, and the performance consequence of false packet detection is that the demodulator may repeatedly spend a large duration of time searching for the data payload part of a packet whose packet detection was initiated by the noise, undergoing a timeout, and then repeating the process with the same noise or interferer which causes the false packet detection. During these durations of time which follow the false packet detection, actual packets may be ignored because the receiver is fruitlessly attempting to demodulate a packet from noise which triggered the packet detect circuit.
One problem of the prior art is the time delay in packet detection which results from performing AGC operations during the preamble interval prior to packet detection. For the case of a preamble having n repeating patterns, a total of n−1 preamble detection events is available, since one preamble is typically loaded into the correlator before detection may occur. If the preamble AGC time spans m such repeating pattern time intervals, there may only be n−m−1 preamble detection events, which may result in loss of preamble detection for a packet with a short preamble such as 802.11g.
Another problem of the prior art is that high and low signal to noise packets are typically handled the same way by the packet acquisition systems.